This book-in-progress examines concerns over "environmental health" in American history through a detailed study of one locality over the course of a century and a half (1840-1990), illustrating how material changes in a particular landscape, in combination with broader changes in medicine and culture, helped drive the emergence of environmental health concerns and local environmental movements. Until the twentieth century, Europeans and Americans saw new lands as potentially dangerous to their own health, and concerns over health moved settlers to analyze the disease-generating and health-giving qualities of particular regions and environmental features. Today Westem attitudes have been substantially reversed. The natural environment is seen as intrinsically healthy; yet it can be made unhealthy through human use and abuse. This book traces that perceptual and material transformation for one locality in Western North America, the Central Valley of California - showing how a landscape that was understood to be naturally dangerous came to be understood as naturally benign, and then unnaturally dangerous. By necessity, this work engages environmental history with the history of medicine and public health. The manuscript is based primarily on archival research. Sources for understandings perceptions of environment and health include professional publications, published and unpublished materials of the California Department of Public Health, the U.S. Public Health Service, and certain key individuals, local newspapers, and legislative testimony. The environmental history of the region is reconstructed from engineering and environmental studies, agricultural literature, and early environmental surveys.